Adherence to basic principles of physiologic horseshoeing are
essential in maintaining hoof health and soundness. Most
horses do not require special trimming or shoeing techniques.
Becoming familiar with a few basic concepts can help the
veterinarian recognize when changes in trimming and/or shoeing
might be expected to help the performance of a sound horse or
to help restore the performance of one that is lame.

Introduction

There may be no other routine procedure performed on the
equine athlete that has more influence on soundness than hoof
preparation and shoeing. Physiological horseshoeing could be
defined as that which promotes a healthy functional foot,
biomechanical efficiency and prevents lameness 1.
As the veterinarian is responsible for the total care of the
horse, a working knowledge of farriery is essential.

Trimming and shoeing can affect a variety of important
parameters, including the manner in which the foot lands, the
duration of the stance phase of the stride and breakover. In
addition to affecting normal foot function, injuries related
to landing and weight bearing can be influenced by trimming
and shoeing. A thorough knowledge of proper traditional
horseshoeing 1,2,3 enables the veterinarian to
interact with the farrier to enhance and promote quality hoof
care. Important aspects include hoof balance, hoof length and
hoof angulation.

Hoof Balance

The term hoof balance was seldom used in the early farrier
texts. Hoof balance should be considered a concept, as it has
no universal definition. Indeed, when the term "balance the
hoof" is used, multiple questions may arise because hoof
balance can be further divided into geometric, dynamic and
natural balance.4 Unfortunately, it may not be
possible to satisfy all these concepts simultaneously.

Geometric balance, which observes the horse at rest, means the
foot should be symmetrical, i.e. the foot is trimmed so that
the ground surface of the hoof is perpendicular to the long
axis of the limb. While a useful and readily identifiable
concept, the problem with relying on geometric balance alone
is that it does not consider the landing pattern of the foot
nor does it consider any potential relationship between leg
and foot conformation.

Dynamic balance, which observes the horse in motion, implies
that a balanced foot should land symmetrically, i.e. the foot
should land flat with the hope that this places force
uniformly on the solar surface of the hoof wall. The problem
here is that often it is not possible to achieve a flat strike
pattern due to leg conformation. Furthermore, it may be
detrimental to the horse if trimmed to land in a flat strike
pattern if abnormal leg conformation is present.

Recently the term natural balance has been introduced. This
suggests that foot conformation should be modeled after the
foot in its natural state, i.e. feral horses. It is unknown if
this type of balance allows maximum functional strength, it
does not take into account the specific athletic activity of
the horse and it is largely incompatible with traditional
horseshoeing techniques.4

Hoof balance encompasses both dorsal palmar/plantar balance
and mediolateral balance. Improper mediolateral hoof balance
has been associated with foot problems such as sheared heels,
distorted hoof walls and hoof cracks. These problems arise
from disproportionate forces placed on the lateral or medial
aspects on the foot. 5 A problem arises with the
exclusive use of either geometric or dynamic balance to
address mediolateral trimming, as each may produce a foot with
different shape if the landing pattern or limb conformation
deviates from normal. Because of this, no one standard method
of trimming will achieve optimum mediolateral balance for
every horse.

Hoof Length

Hoof length (toe length) is determined by measuring from the
toe at the ground surface proximally to the end of the horny
wall at the coronary band. Guidelines for appropriate toe
length have been established for most breeds based on body
weight.6

Table 1: Guidelines for hoof length based on the weight of the
horse

Horse weight

Toe length

Horse Size

Kg

Pounds

Cm

Inches

Small

360 - 400

800 - 900

7.6

3.0

Medium

425 - 475

950 - 1050

8.25

3.25

Large

525 - 575

1150 - 1250

8.9

3.5

When using these guidelines, it becomes necessary to consider
anatomic variations in the relative position of the third
phalanx within the hoof capsule the thickness of the hoof
wall, hoof shape and the horse's athletic endeavor.

Hoof Angle The angle of the hoof is the angle formed at
the junction of the dorsal hoof wall and the ground surface of
the foot. Until recently, the veterinary and farrier
literature recommended that the normal hoof angle be 45-to-50
degrees for the forefeet and 50-to-55 degrees for the hind
feet. These recommendations have been proven erroneous, as
they do not take into consideration the conformation of the
horse's individual limbs. 7 Ideal hoof angulation
occurs when a line drawn down the dorsal surface of the hoof
wall and a line drawn along the surface of the heel are in
alignment or parallel to a line drawn through the three
phalanges (P1, P2, P3; ).
The foot is trimmed appropriately and the hoof angle is
correct for the individual horse when the dorsal hoof wall and
the dorsal surface of the pastern region are parallel. 7
This is known as the hoof pastern axis (HPA). In order to
confirm this hoof-pastern axis visually, the horse should be
on a hard level surface and must stand squarely on all four
feet with the cannon bones positioned vertically. The terms
low hoof angle and high hoof angle can be used simply to
describe a non-linear relationship between the dorsal hoof
wall and the dorsal pastern region.

In the normal foot, a vertical line drawn from the center of
the lateral condyle of the distal second phalanx to the ground
should bisect the bearing surface of the foot.8 This
line would mark the center of rotation of the distal
interphalangeal joint and would coincide with a line drawn
across the solar surface of the foot through the middle one
third of the frog. This line drawn across the solar surface
should equal the widest part of the foot.

Functional Considerations

Foot conformation (shape) is important because of its
relationship to the foot's biomechanical function. Any changes
made to the bottom of the horse's foot will have an effect on
the angulation of the hoof, the hoof pastern axis and the
alignment of the hoof capsule under the center of rotation.
Variation away from optimum for these parameters may result in
decreased biomechanical efficiency.

Perhaps the most commonly discussed functional element of hoof
movement is breakover. Breakover is defined as the phase of
the stride between the time the horse's heel lifts off the
ground and the time the toe lifts off the ground. The toe acts
as a fulcrum around which the heel rotates under the influence
of the deep digital flexor tendon. The suspensory ligament to
the navicular bone and the impar ligament are under maximal
stress just before breakover.9 Changes in toe
length, hoof/pastern axis and hoof angle all affect breakover
and the tensile forces on the deep digital flexor tendon.
However, it may not be possible to predict what changes in
breakover will occur as a result of a particular trimming
modification, nor will such changes necessarily occur at all
gaits. In general, breakover is significantly delayed with the
presence of a long toe and acute hoof angle because the long
toe acts as a long lever arm, requiring more time and forces
to rotate the heel around the toe. In addition, it is felt
that tension exerted by the deep digital flexor tendon against
excessive toe length results in lamina tearing, which may lead
to hoof distortion.

Low hoof angles, where the angle of the dorsal hoof wall is
lower than the angle of the dorsal pastern, create a
broken-back hoof pastern axis. This type of foot configuration
is commonly caused by the long-toe/underrun-heel foot
conformation. A low hoof angle causes coffin joint extension,
increased strain on the deep digital flexor tendon and
promotes toe-first landing. This, in turn, may cause increased
stress on the soft tissue structures associated with the
navicular bone and may delay the speed of breakover. There is
experimental evidence that a low hoof angle will compromise
circulation in the heel area of the foot.10 This
abnormal hoof conformation is known to contribute to navicular
syndrome, chronic heel pain (bruising), coffin joint
inflammation, quarter and heel cracks and interference
problems.

High hoof angles, where the angle of the dorsal hoof wall is
higher than the angle of the dorsal pastern, create a
broken-forward hoof pastern axis. An extremely high hoof angle
is often classified as a "club foot." Some horses with
extremely upright pasterns may be falsely identified as having
a clubfoot. A high hoof angle causes coffin joint flexion,
promotes heel-first landing and increases pressure in the
heel. Some injuries associated with a high hoof angle are
coffin joint inflammation due to abnormal loading of this
joint, sole bruising and increased strain on the suspensory
ligaments of the navicular bone.

Objectives of Trimming

The goal of trimming and shoeing the equine foot is to
facilitate breakover, ensure solar protection and provide
palmar/plantar heel support.

Facilitating breakover, i.e., moving breakover in a palmar/plantar
direction, shortens the lever arm created by the toe, changes
the tensile forces in the deep digital flexor tendon, moves
the navicular bone slightly proximal and changes the angle
between P2 and P3. Decreasing the toe length through trimming
can facilitate breakover, as can applying a rolled toe,
rockered toe or square toe shoe.

The function of the sole is to protect and support the
underlying structures, and to bear some weight around its
border with the hoof wall. The normal sole should be firm on
digital pressure and concave. Flattened soles should be
considered abnormal and may have a pathological cause.10
This concavity increases the sole's weight bearing capacity.
Sufficient sole depth is necessary to maintain this concavity
and to protect the underlying dermis, its circulation and the
third phalanx from injury. Inadequate sole depth is the most
common cause of chronic subsolar bruising.11 Sole
depth can be maintained by trimming back the toe so that the
dorsal hoof wall is aligned and by not removing excess live
sole with the hoof knife. Placing a pad between the shoe and
foot to prevent abrasion to the sole from the ground can
further protect the sole.

It is important to provide palmar/plantar support to the foot
because of the forces placed on the heels during the landing
and stance phase of the stride. In the ideal situation, an
imaginary line that bisects the third metacarpal bone should
intersect the most palmar extent of the ground surface of the
wall.12 The farrier literature has purported that
it is beneficial and necessary to trim the heels to the widest
part of the frog at the heel in order to support this area.
This may not be accurate in many cases. It may not be helpful
or even possible to trim a low heel or underrun heel in this
manner. On the other hand, a heel may be trimmed excessively
to reach the desired ground surface at the expense of
affecting the angulation of the foot or the parallelism
between the dorsal hoof wall and the pastern. If the heel
cannot be trimmed to provide optimal ground surface, the
branch of the shoe can be extended to compensate for this lack
of bearing surface.

Method of Hoof Preparation and Shoeing

Farriery can be divided into three major parts, trimming the
foot and fitting and attaching the shoe.

a). Trimming

Trimming is the most important aspect of horseshoeing as it
creates the base to which a shoe is attached. Before preparing
the foot, the farrier should visually examine its conformation
by viewing it from the front, the side and from behind the
standing horse. The horse should also be walked toward and
away from the farrier so that he can observe foot placement.
The two useful visual references when trimming are the hoof
pastern axis and the widest part of the foot which is located
under the center of rotation. If questionable distortions are
present in the hoof capsule, radiographs can be taken and used
for guidance while trimming.

The foot is lifted off the ground, held naturally at the
fetlock and the farrier's head is positioned directly above
the foot to judge the mediolateral orientation of the foot and
what adjustments need to be made while trimming. The frog,
sole and bars are only trimmed where necessary to remove
loose, exfoliating material. The base of the frog is trimmed
next to both sulci to enhance cleaning. The sole is pared at
the sole wall junction (white line) to determine the amount of
hoof wall to be removed. Excess wall is removed using hoof
nippers, beginning the cut in a tapered fashion just in front
of the heels and continuing around the circumference of the
foot to the opposite quarter. Mediolateral orientation is
again checked and the foot is rasped in a circular manner
blending the heels into the original cut created by the hoof
nippers. The heels are rasped according to the foot
conformation or until the heel, angle of the sole and bar form
a solid base. The heels should not be trimmed below the ground
surface of the frog. Ideally, the frog should be about level
with the ground surface of the wall at the heels. The dorsal
hoof wall at the toe should be "backed up " with a rasp to
one-half the thickness of the hoof wall or as far as the sole
wall junction if necessary to align the dorsal hoof wall. A
thin layer of sole is removed at the sole wall junction to
ensure that no sole pressure exists.

b). Fitting the shoe

The shoe used should be the lightest and simplest possible
that provides traction, protection, and adequate support to
the foot for the work being performed. The shoe is an
extension of the properly trimmed foot; therefore it should be
accurately fitted to the outline of the prepared hoof wall.
Breakover can be modified by broadening the toe of the shoe
and bring the leading edge of the shoe back to, but not
behind, the white line. Expansion is necessary in the heel
area to allow for the natural elasticity and movement of the
hoof wall at the heels. The length of the shoe should be long
enough to cover the buttress of the heel and to support the
leg. If we consider the dorsal third of the frog or the widest
part of the foot to be under the center of rotation, then this
area should be equidistant to the toe and to the heels of the
shoe. If these distances are not equal in the unshod foot, the
shoe can be lengthened accordingly at the heels. If heel angle
is low, it can be raised through the use of swelled heel shoes
or wedge heel inserts. Wedge pads will mechanically lift the
heel and give the impression of correction, but the correction
will be short lived. Raising the heels using wedge pads
results in increased pressure on the hoof wall at the heels
and the supporting structures in this area. This tends to
encourage more heel wear and eventual collapse.

c). Applying the Shoe

While advances in adhesives may ultimately make horseshoe
nails obsolete; they are still the most common method of
applying a shoe. As such, there is not enough emphasis placed
on the proper use of nails. The concept of nailing is to use
the fewest nails possible and the smallest nail that will hold
the shoe securely in place during the shoeing interval. Nail
hole location is important so as not to interfere with the
movement of the foot. This means nail placement should be
forward of the widest part of the foot. Machine shoes in
common use today are made with no angle or slope to the nail
holes. Repunching the nail holes at an angle allows the nails
to be placed in solid hoof wall prevents the hoof wall from
splitting which can lead to movement of the shoe.

Discussion

The importance or proper farriery is obvious if we bear in
mind that what is done to the external hoof through trimming
and shoeing affects the internal foot structures as well as
the limb above, Most veterinarians and farriers agree that a
large proportion of the lameness seen today could be prevented
or treated through good farriery. The horse should have a
foot/shoe configuration that matches its size, conformation
and limb motion as well as its athletic endeavor. Sound
physiological horseshoeing can only be achieved by a thorough
knowledge of, strict adherence to, and the skillful
application of basic principles such as hoof angle, hoof
pastern axis and mediolateral balance. Only then does farrier
science truly become an art form.